Automotive active suspension system for regulating vehicular height level during anti-rolling control

ABSTRACT

An active suspension system is provided which includes hydraulic cylinders for suppressing rolling motion of a vehicle body. The system is responsive to lateral acceleration acting on the vehicle body to provide a control signal for supplying hydraulic pressure to the cylinders for anti-rolling control. The control signal includes first and second pressure control signals. The first pressure control signal commands supply of hydraulic pressure for providing an anti-rolling moment, while the second pressure control signal commands supply of hydraulic pressure for supporting a portion of the static load of the vehicle body to maintain the vehicle body at a target height level. The system is further responsive to increase in lateral acceleration to modify the second pressure control signal to reduce the hydraulic pressure for supporting the portion of static load, thereby compensating for forces acting to raise the vehicle body caused by the anti-rolling control for establishing a constant vehicular height level irrespective of variation in the lateral acceleration acting on the vehicle body.

BACKGROUND OF THE INVENTION Field of The Invention

The present invention relates generally to an active suspension controlsystem for a vehicle for suppressing vehicular rolling motion caused bylateral acceleration acting on a vehicle body. More specifically, theinvention relates to an active suspension control system which isoperable to adjust a vehicular height level during anti-rolling motioncontrol.

Description of The Background Art

Japanese Patent First Publication No. 62-295714, which corresponds tothe U.S. application No. 07/657,946, discloses an active suspensioncontrol system for a vehicle. This prior art suspension control systemincludes generally hydraulic cylinders disposed between suspensionmembers and a vehicle body and coil springs arranged parallel thereto sothat internal pressures of the hydraulic cylinders are maintained at apreselected neutral pressure when no vehicular rolling motion occurs toprovide thrusts required for supporting a portion of static load of avehicle. When the vehicle is turned to cause rolling motion to occur,the system detects lateral acceleration acting on the vehicle body toraise the pressures in the hydraulic cylinders for the outside wheelswith the pressures in the hydraulic cylinders for the inside wheelsbeing reduced, based on an amount of the detected lateral acceleration,for generating an anti-rolling moment which compensates a rolling momentcaused by lateral vehicular load shift created proportional to thelateral acceleration acting on the vehicle body.

Referring to FIG. 1, a three-link model is shown which shows a vehiclebody and right and left suspensions by way of explaining forces actingon a vehicle body from a road surface during turning in theabove-described conventional active suspension system.

On a surface of a tire contacting with a road surface, force is createdto bear lateral displacement of the vehicle body to be balanced withcentrifugal force. From this contact surface, reaction force orcornering force is generated in a direction of the rolling motioncenter. A vertical component of the reaction force against an outsidewheel during turning acts as a jack-up force W_(U), raising the vehiclebody, while a vertical component at an inside wheel acts as a jack-downforce W_(D), lowering the vehicle body. During turning, due to lateralload shift of the vehicle body, a vertical load acting on the outsidewheel is increased while a vertical load acting on the inside wheel isdecreased with the result that a cornering force C_(FO), created on theoutside wheel, becomes greater than cornering force C_(FI), created onthe inside wheel. Consequently, if the rolling motion center is, asshown in the drawing, positioned above the road contact point of thetires, and the link angles of the left and right links are substantiallymaintained at the same values caused by anti-rolling control, thejack-up force W_(U) becomes greater than the jack-down force W_(D). Theresultant additional jack-up force then acts to raise the vehicle body.This vehicle body raising force is increased according to increase inlateral acceleration during turning. It will be noted that due to theabove mentioned jack-up force raising a vehicular height level duringturns, a stroke of an outside wheel suspension becomes greater than adesired value to vary a camber and/or toe angle. Additionally, when avehicle travels on an uneven road, the outside wheel suspension isextended completely to degrade the gripping ability of a tire. When thevehicle is turned at a high speed and great lateral acceleration acts onthe vehicle, the vehicle body rises further to also degrade gripping ofa tire of the inside wheel. This results in driving instability.

For avoiding the above drawbacks, improving link arrangements of atypical mechanical suspension may be proposed to move the rolling motioncenter downward in a turning inside direction according to rollingmotion of a vehicle body. However, an active suspension systemsuppresses vehicular rolling motion practically, therefore, shifting therolling-motion center to prevent the vehicle body from rising is notpractically accomplishable in such systems.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to avoid thedisadvantages of the prior art.

It is another object of the invention to provide an active suspensioncontrol system which serves to compensate a force raising a vehiclebody, caused by anti-rolling motion control during turning, forestablishing suitable driving stability.

According to one aspect of the present invention, there is provided anactive suspension system for a vehicle comprising: suspension unitsincluding actuators disposed between a vehicle body and suspensionmembers which respectively support wheels rotatably, each actuator beingadjustable of fluid pressure therein for active suspension control; apressure source which supplies fluid pressure to the actuators of thesuspension units; pressure control means for controlling the fluidpressure supplied from the pressure source to the actuators of thesuspension units respectively; sensor means for detecting lateralacceleration acting on the vehicle body to provide a signal indicativethereof; and control means responsive to the signal from the sensormeans to provide a control signal to the pressure control means whichincludes first and second pressure control signals, the first pressurecontrol signal commanding the pressure control means to provide firstfluid pressure for suppressing rolling motion caused by the lateralacceleration acting on the vehicle body, the second pressure controlsignal commanding the pressure control means to provide second fluidpressure for establishing a preselected vehicular height level, thecontrol means being responsive to increase in the lateral accelerationdetected by the sensor means to correct the second control signal forreducing the second fluid pressure by a preselected rate.

According to another aspect of the present invention, there is providedan active suspension system for a vehicle comprising: suspension unitsincluding actuators disposed between a vehicle body and suspensionmembers which respectively support wheels rotatably, each actuator beingadjustable of fluid pressure therein for active suspension control; apressure source which supplies fluid pressure to the actuators of thesuspension units; pressure control means for controlling the fluidpressure supplied from the pressure source to the actuators of thesuspension units respectively; sensor means for detecting lateralacceleration acting on the vehicle body to provide a signal indicativethereof; and control means responsive to the signal from the sensormeans to provide a control signal to the pressure control means to varythe fluid pressure to the actuators by a preselected rate according tovariation in the lateral acceleration acting on the vehicle body forproviding anti-rolling moment, the control means being furtherresponsive to increase in the lateral acceleration to correct thecontrol signal so as to reduce the hydraulic pressure to the actuatorsfor compensating for forces caused by the lateral acceleration acting onthe vehicle body to cause the vehicle body to rise above a preselectedheight level to maintain the vehicle body at the preselected heightlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an illustration which shows a three-link model by way ofexplaining forces from a road surface acting on a vehicle body duringturning.

FIG. 2 is a schematic view which shows an active suspension systemaccording to the present invention.

FIG. 3 is a graph which shows the relationship between output pressureof a pressure control valve and current applied thereto.

FIG. 4 is a graph which shows the relationship between lateralacceleration acting on a vehicle body and a value of detected lateralacceleration.

FIG. 5 is a block diagram which shows a control unit of an activesuspension system of the invention.

FIG. 6 is a graph which shows the relationship between detected lateralacceleration acting on a vehicle body and an output value of a neutralpressure setting circuit of an active suspension system according to theinvention.

FIG. 7 is a graph which shows the relationship between detected lateralacceleration acting on a vehicle body and pressure created in ahydraulic cylinder for a front wheel.

FIG. 8 is a graph which shows the relationship between detected lateralacceleration acting on a vehicle body and pressure created in ahydraulic cylinder for a rear wheel.

FIG. 9 is a graph which shows the relationship between detected lateralacceleration acting on a vehicle body and pressure created in ahydraulic cylinder for a front wheel in a conventional active suspensionsystem.

FIG. 10 is a graph which shows the relationship between detected lateralacceleration acting on a vehicle body and pressure created in ahydraulic cylinder for a rear wheel in a conventional active suspensionsystem.

FIG. 11 is a schematic view which shows an laternative active suspensionsystem according to the present invention.

FIG. 12 is a block diagram which shows a control unit of an activesuspension system in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIG. 1, an activesuspension system 16 for a vehicle according to the present invention isshown. This system is operable to effect suspension control forregulating a vehicle height level and vehicle attitude change bysuppressing relative displacements between a vehicle body 14 andsuspension members 12 respectively which support front-left,front-right, rear-left, and rear-right wheels 10FL, 10FR, 10RL, and10RR.

The active suspension system 16 includes generally four suspension units15FL, 15FR, 15RL, and 15RR for the corresponding wheels. The suspensionunits include working fluid cylinders 18FL to 18RR such as a hydrauliccylinder, functioning as an actuator, disposed between the suspensionmembers 12 and the vehicle body 14, coil springs 36 arranged parallel tothe hydraulic cylinders, and pressure control valves 20FL to 20RR whichserve to adjust hydraulic pressure supplied to the hydraulic cylinders18FL to 18RR respectively. The coil springs 36 have a relatively lowspring constant for bearing a portion of static load of the vehicle body14.

The active suspension system 16 further includes a pressure source 22,two accumulators 24, a lateral acceleration sensor 26, and a controlunit 30. The accumulators 24 are arranged between the pressure source 22and the front pressure control valves 20FL and 20FR and the rearpressure control valves 20RL and 20RR respectively for accumulatinghydraulic pressure from the pressure source 22. The lateral accelerationsensor 26 monitors lateral acceleration acting on the vehicle body 14 toprovide a signal indicative thereof to the control unit 30. The controlunit 30 is then responsive to the signal from the lateral accelerationsensor 26 to modify hydraulic pressure output from the pressure controlvalves 20FL-to 20RR to the hydraulic cylinders 18FL to 18RRindependently for controlling rolling motion of the vehicle body 14.

The suspension units 15FL to 15RR further include throttles, or orifices32 communicating with pressure chambers L, as will be describedhereinafter in detail, of the hydraulic cylinders 18FL to 18RR andaccumulators 34 for absorbing vibration having a relatively highfrequency transmitted from the wheels respectively.

Each of the hydraulic cylinders 18FL to 18RR includes a cylinder tube18a which defines an upper pressure chamber L closed by a piston 18c. Anupper portion of the cylinder tube 18a is attached to the vehicle body14, while a lower portion of a piston rod 18b is attached to thesuspension member 12.

Each of the pressure control valves 20FL to 20RR is designed as athree-port proportional electromagnetic pressure reducing valve whichincludes supply, return, and output ports. For example, U.S. Pat. No.4,967,360, issued on Oct. 30, 1990 and assigned to the applicant commonto the present invention, discloses a valve of this type, disclosure ofwhich is incorporated herein by reference. The pressure control valveincludes a valve housing having a cylindrical valve bore in which avalve spool is slidably disposed and a proportional solenoid installedin the valve housing. The supply and return ports are communicated withhydraulic pressure supply and return ports of the pressure source 22through hydraulic lines 38 and 39 respectively for supply and returnhydraulic pressure to and from the pressure control valve, while theoutput port is communicated with the pressure chamber L of the hydrauliccylinder through a hydraulic line 40.

With these arrangements, adjustment of a command, or exciting, current iapplied to the solenoid causes a displacement of the valve spool to becontrolled, thereby causing control pressure P_(C), to be output fromthe output port of the pressure control valve 20FL (-20RR) to thepressure chamber L of the hydraulic cylinder 18FL (-18RR), to becontrolled according to the magnitude of the exciting current i.

Referring to FIG. 3, the relationship between the exciting current i andcontrolled pressure P_(C) is shown. When the exciting current i isapproximately zero, the pressure control valve provides a minimumcontrol pressure P_(MIN). As the exciting current is positivelyincreased from this level, the control pressure P_(C) becomes greatproportionally with a preselected gain K₁ and is then saturated at amaximum set pressure P_(MAX) of the hydraulic pressure source 22.Additionally, when the exciting current of i_(N) is provided accordingto a neutral pressure command value VN₀ as will be described hereinafterin detail, the pressure control valve outputs neutral pressure P_(CN).

Referring to FIG. 4, the relationship between lateral accelerationacting on the vehicle body 14 and a value G of lateral accelerationdetected-by the lateral acceleration sensor 26 is shown. When a vehicleis turned to the right, the lateral acceleration sensor provides asignal indicative of a lateral acceleration detected value G having apositive voltage proportional to lateral acceleration acting on thevehicle body, while when the vehicle is turned to the left, a signalhaving a negative voltage is provided.

Referring to FIG. 5, the control unit 30 includes generally neutralpressure setting circuits and 50R, variably adjusting gain circuits forthe front and rear suspension units 52F and 52R, adder circuits 54FL,54RL, 54FR, and 54RR, and driving circuits 58FL to 58RR.

The neutral pressure setting circuits 50F and 50R are responsive to asignal indicative of a lateral acceleration detected value G from thelateral acceleration sensor 26 to provide neutral pressure commandvalues VN_(F) and VN_(R) having voltage according to the lateralacceleration detected value. The variably adjusting gain circuits 52Fand 53R are also responsive to the signal from the lateral accelerationsensor 26 to multiply the lateral acceleration detected value G byanti-rolling control gains K_(F) and K_(R) for the front and rearsuspension units to provide anti-rolling command values VL_(F) andVL_(R) respectively. These anti-rolling command values VL_(F) and VL_(R)are then respectively inputted to one of input ports of the addercircuits 54FL and 54RL directly while, at the other input ports of theadder circuits 54FL and 54FR, the neutral pressure command values VN_(F)and VN_(R) from the neutral pressure setting circuits 50F and 5OR, areinput. Additionally, the anti-rolling command values VL_(F) and VL_(R)are further input to one of input ports of the adder circuits 54FR and54RR with reversed polarities, converted by inverters 56F and 56Rrespectively, while to the other input ports, the neutral pressurecommand values VN_(F) and VN_(R) from the neutral pressure settingcircuits 50F and 50R are input. The adder circuits 54FL to 54RR outputthe added command values to the driver circuits 58FL to 58RRrespectively. The driver circuits 58FL to 58RR are designed as afloating type constant-voltage circuit for example which provideexciting currents i_(FL) to i_(RR) to the proportional solenoids of thepressure control valves 20FL to 20RR according to the added commandvalues respectively.

The neutral pressure setting circuits 50F and 50R provide neutralpressure command value correcting means respectively which may include afunction generator operable to output the neutral pressure commandvalues VN_(F) and VN_(R) according to the lateral acceleration detectedvalue G.

The above control unit 30 may be provided with a micro-computer whichoperates arithmetically.

Referring to FIG. 6, there is shown the relationship between lateralacceleration acting on the vehicle body and signal values output fromthe neutral pressure setting circuits 50F and 50R. The neutral pressuresetting circuit 50F for the front suspension units is responsive to thelateral acceleration detected value G of zero to provide a preselectedpositive value VN₀ for supporting part of a static load of the vehiclebody. As the lateral acceleration detecting value G is positively ornegatively varied from this level, the neutral pressure command valueVN_(F) is outputted which is decreased gradually by a first preselectedrate as shown by a solid line in the drawing. Additionally, the neutralpressure setting circuit 50R for the rear suspension units outputs thesame preselected positive value VN₀ as that of the neutral pressuresetting circuit 50F for supporting part of the static load of thevehicle body when the lateral acceleration detected value G is zero. Asthe lateral acceleration detected value G is positively or negativelyvaried from this level, the neutral pressure command value VN_(R) isoutput which is decreased gradually by a second preselected rate smallerthan the first preselected rate as shown by a broken line. In otherwords, the neutral pressure setting circuit 50F for the front suspensionunits is responsive to increase of lateral acceleration acting on thevehicle body to provide the neutral pressure command value VN_(F), whichis smaller than the value VN_(R), for the rear suspension units.

A total gain K (=K_(F) +K_(R)) of the gains K_(F) and K_(R) of thevarlably adjusting gain circuits 52F and 52R for the front and rearsuspension units is provided to establish vehicle rolling motion ofabout zero while lateral acceleration acts on the vehicle body.Additionally, the gain K_(F) is set to a value greater than that of thegain K_(R) (K_(F) >K_(R)) in this embodiment. Thus, a lateral loaddisplacement between the front left and right wheels caused byanti-rolling control when lateral acceleration acts on the vehicle bodybecomes great relative to a lateral load displacement between the rearleft and right wheels so that vehicle steering characteristics is set inan understeering direction.

The driving circuits 58FL to 58RR output exciting currents i determinedbased on values output from the adder circuits 54FL to 54RR to thepressure control valves 20FL to 20RR respectively for adjustinghydraulic pressure supplied to the hydraulic cylinders 18FL to 18RR.

In operation, assuming that a vehicle is traveling straight at aconstant speed on an even road without any protrusions, the lateralacceleration sensor 26 provides a signal representing the lateralacceleration detected value G of zero which indicates no lateralacceleration acting on the vehicle body. The neutral pressure settingcircuits 50F and 50R of the control unit 30 are then responsive to thesignal from the lateral acceleration sensor 26 to provide the neutralpressure command values VN_(F) and VN_(R) indicative of the preselectedpositive value VN₀ respectively as shown in FIG. 6. In addition, thevariably adjusting gain circuits 52F and 52R provide the anti-rollingpressure command values VL_(F) and VL_(R) with a value of zerorespectively. Thus, all the adder circuits 54FL to 54RR output thepreselected value VN₀ as a result of adding to the driving circuits 58FLto 58RR. The driving circuits 58FL to 58RR then output exciting currents1FL to 1RR as the neutral pressure command current i_(N) as shown inFIG. 3. The pressure control valves 20FL to 20RR are then responsive tothe neutral pressure command currents i_(N) respectively to output thecontrolled pressure P_(C) indicating the neutral pressure P.sub. CN.Internal pressures of the hydraulic cylinders 18FL to 18RR are alsomodified to the neutral pressure P_(CN), thereby maintaining the vehiclebody on a level orientation at a target vehicle height.

When the vehicle is turned from a straight traveling status, the vehiclebody tends to roll and the lateral acceleration sensor 26 detectslateral acceleration acting on the vehicle body to provide a signalindicating a lateral acceleration detected value G, having a positive ornegative level, to the control unit 30. The neutral pressure settingcircuits 50F and 50R of the control unit 30 then decrease the neutralpressure command values VN_(F) and VN_(R) output therefrom respectivelyby different rates, as described above, according to an increase in thelateral acceleration detected value G.

Additionally, the varlably adjusting gain circuits 52F and 52R for thefront and rear suspension units are responsive to the signal from thelateral acceleration sensor 26 to multiply the lateral accelerationdetected value G by the anti-rolling control gains K_(F) and K_(R) andthen provide the anti-rolling command values VL_(F) and VL_(R).

Accordingly, if the vehicle is now turned to the right, the lateralacceleration detected value G represents a positive level. The addercircuit FL, therefore, adds the positive anti-rolling control commandvalue VL_(F) to the positive neutral pressure command value VN_(F) toprovide a value which is greater than the value VN_(F) by the valueV_(LF). In contrast, the adder circuits 54FR provides a value which issmaller than the neutral pressure command value VN_(F) by theanti-rolling control command value VL_(F) since the VL_(F) supplied tothe adder circuit 54FR is inverted in polarity by the inverter 56F.Similarly, the adder circuit 54RL provides a value which is greater thanthe neutral pressure command value VN_(R) by the anti-rolling controlcommand value VL_(R), while the adder circuit 54RR provides a valuewhich is smaller than the neutral pressure command value VN_(R) by theanti-rolling control command value VL_(R). With these values output fromthe adder circuits, the pressure control valves 20FL and 20RLrespectively output a controlled pressure P_(C), greater than theneutral pressure P_(CN), to the hydraulic cylinders 18FL and 18RL forthe front left and rear left wheels (the outside wheels during rightturning), causing pressures therein to be elevated to provide thrustsagainst vehicular rolling motion. The pressure control valves 20FR and20RR respectively output a controlled pressure P_(C) smaller than theneutral pressure P_(CN) to the hydraulic cylinders 18FR and 18RR for thefront right and rear right wheels (the inside wheels during rightturning), causing pressures therein to be reduced to provide thrustswhich do not promote the rolling motion. This results in the vehiclebody being maintained at a flat, level orientation.

The neutral pressure command values VN_(F) and VN_(R) output from theneutral pressure setting circuits 50F and 50R, as previously mentioned,are decreased according to increase in the lateral accelerationdetecting value G, therefore, the neutral pressures P_(NF) and P_(NR)corresponding to the values VN_(F) and VN_(R) are also reduced as shownin FIGS. 7 and 8. Thus, the controlled pressure P_(CFL) increased by thepressure control valve 20FL for the front outside wheel and thecontrolled pressure P_(CFR) decreased by the pressure control valve 20FRfor the front inside wheel vary, as indicated by solid and broken linesin FIG. 7, according to the variation in the neutral pressure P_(NF) inopposite directions by the same amount (this amount, as mentioned above,corresponds to the anti-rolling pressure command value VL_(F)) withrespect to a value of the neutral pressure P_(NF). In control for therear suspension units, the neutral pressure P_(NR) varies by a ratesmaller than that of the neutral pressure P_(NF) for the frontsuspension units as shown in FIG. 8. Thus, the controlled pressureP_(CRL) increased by the pressure control valve 20RL for the rearoutside wheel and the controlled pressure P_(CRR) decreased by thepressure control valve 20RR for the rear inside wheel vary, as indicatedby solid and broken lines in the drawings, according to the variation inthe neutral pressure P_(NR) by rates smaller than those of the P_(CFL)and P_(CFR).

It will be appreciated that the average pressure of the left and righthydraulic cylinders (corresponding to the neutral pressure) is decreasedaccording to increase in lateral acceleration acting on the vehiclebody, thereby reducing thrusts provided by the hydraulic cylinders 18FLto 18RR for supporting part of a static load of the vehicle body tocompensate for the increased jack-up force W_(U) as mentionedpreviously. Therefore, vehicular rolling motion is suitably suppressedand height of the center of gravity of the vehicle body is maintained ata preselected constant level during turning, irrespective of variationin lateral acceleration acting on the vehicle body.

Referring to FIGS. 9 and 10, by way of explaining a prior art system,since the prior art system maintains the neutral pressure P_(CN) at aconstant level regardless of variation in lateral acceleration acting ona vehicle body, output pressures P_(CFL) and P_(CRL) for the left wheelsfrom the hydraulic cylinders 18FL and 18RL and output pressures P_(CFR)and P_(CRR) for the right wheels from the hydraulic cylinders 18FR and18RR, vary by the same rates respectively with respect to the neutralpressures P_(NF) and P_(NR) dependent upon variation in the anti-rollingcontrol gains K_(F) and K_(R) for the front and rear suspension units.With this control, jack-up force W_(U) acting on the outside wheelsduring turning is increased to cause the vehicle body to rise, therebyincreasing height of the center of gravity of the vehicle body accordingto increase in lateral acceleration acting on the vehicle body.

Referring to FIG. 11, an alternative active suspension system accordingto the present invention is shown. As discussed above, the magnitude ofjack-up forces created at front and rear wheels depend on lateralaccelerations acting on the front and rear wheels respectively.Accordingly, the active suspension system as discussed below serves todetermine lateral accelerations acting on the front and rear wheelsseparately to control neutral pressures supplied to front and rearhydraulic cylinders independently to suppress jack-up forces completelyin a transitional status during turning where different lateralaccelerations act on the front and rear wheels.

The shown active suspension system is different from the above mentionedfirst embodiment in that two sensors, or front and rear lateralacceleration sensors 26a and 26b are provided. Other arrangements aresimilar to the first embodiment, therefore, explanation thereof indetail will be omitted here.

The front lateral acceleration sensor 26a is mounted on a preselectedportion of a vehicle body 14 which is located frontward from the centerof gravity of the vehicle body by a predetermined distance L_(A), forexample, it may be installed on a portion adjacent a front axle andserves to detect lateral acceleration acting on a front side of thevehicle body to provide a signal representing a lateral accelerationdetected value G_(A) to a control unit 30. The rear lateral accelerationsensor 26b is mounted on a preselected portion of the vehicle body 14which is situated rearward from the center of gravity by a predetermineddistance L_(B), for example, it may be installed on a portion adjacent arear axle and serves to detect lateral acceleration acting on a rearside of the vehicle body to provide a signal indicative of a lateralacceleration detected value G_(B) to the control unit.

Referring to FIG. 12, the control unit 30 of the above alternativeembodiment is shown. The control unit 30 includes a lateral accelerationdetermining circuit 55 (G calculator). The lateral accelerationdetermining circuit 55 is responsive to the signals from the front andrear lateral acceleration sensors 26a and 26b to determine a lateralacceleration value G_(Y) indicative of the magnitude of lateralacceleration acting on the center of gravity of the vehicle body, alateral acceleration value G_(F) indicative of the magnitude of lateralacceleration acting on a front wheel axle shaft, and a lateralacceleration value G_(R) indicative of the magnitude of lateralacceleration acting on a rear wheel axle shaft. The lateral accelerationdetermining circuit 55 then outputs a signal indicating the lateralacceleration value G_(Y) to front and rear varlably adjusting gaincircuits 52F and 52R, a signal indicating the lateral acceleration valueG_(F) to a neutral pressure setting circuit 50F, and a signal indicatingthe lateral acceleration value G_(R) to a neutral pressure settingcircuit 50R respectively.

The lateral acceleration determining circuit 55 projects the lateralacceleration values G_(F), G_(R), and G_(Y) with the following equationsbased on the lateral acceleration detected values G_(A) and G_(B) inputthereto.

    G.sub.F ={(L.sub.B -L.sub.F)/(L.sub.B -L.sub.A)}G.sub.A -{(L.sub.A -L.sub.F)-(L.sub.B -L.sub.A)}G.sub.B

    G.sub.R ={(L.sub.B -L.sub.R)/(L.sub.B -L.sub.A)}G.sub.A -{(L.sub.A -L.sub.R)-(L.sub.B -L.sub.A)}G.sub.B

    G.sub.Y =(L.sub.B.G.sub.A +L.sub.A.G.sub.B)/(L.sub.B -L.sub.A)

where L_(A) is a distance between the center of gravity of a vehiclebody and a position where the front lateral acceleration sensor 26a ismounted, L_(B) is a distance between the center of gravity and aposition where the rear lateral acceleration sensor 26b is mounted,L_(F) is a distance between the center of gravity and a front axle andL_(R) is a distance between the center of gravity and a rear axle.

The above equations are derived by the following relations; a positionin front of the center of gravity of a vehicle body is positive, aposition rearward of the vehicle center of gravity is negative, and φ isa yaw rate.

    G.sub.A =G.sub.Y +L.sub.A φ                            (1)

    G.sub.B =G.sub.Y +L.sub.B φ                            (2)

from (1)×L_(B) -(2)×L_(A)

    L.sub.B G.sub.A -L.sub.A.G.sub.B =(L.sub.B -L.sub.A)G.sub.Y thus, G.sub.Y ={1/(L.sub.B -L.sub.A)}(L.sub.B. G.sub.A -L.sub.A.G.sub.B)(3)

from (1)-(2)

    G.sub.A -G.sub.B =(L.sub.A -L.sub.B) φ thus, φ={1/(L.sub.A -L.sub.B)}(G.sub.A -G.sub.B)                              (4)

    G.sub.F =G.sub.Y +L.sub.F φ

    G.sub.R =G.sub.Y +L.sub.R φ

substituting (3) and (4) for the above equations, ##EQU1##

The lateral acceleration value G_(Y) may be lateral acceleration at aportion other than the center of gravity of the vehicle body.

The varlably adjusting gain circuits 52F and 52R are responsive to thesignal indicating the lateral acceleration value G_(Y) to provideanti-rolling command values VL_(F) and VL_(R) respectively in the samemanner as described in the first embodiment. The neutral pressuresetting circuits 50F and 50R are also responsive to the signalsindicative of the lateral acceleration values G_(F) and G_(R) to provideneutral pressure command values VN_(F) and VN_(R).

With the above arrangement, it will be noted that the varlably adjustinggain circuits 52F and 52R serve to provide anti-rolling moment throughthe front and rear hydraulic cylinders 18FL to 18RR based on lateralacceleration acting on the center of gravity of a vehicle body, theneutral pressure setting circuits 50F and 50R serve to reduce neutralpressures supplied to the front and rear hydraulic cylinders closely aslateral accelerations acting on front and rear axles increaserespectively. Consequently, even when lateral acceleration acting on thefront and rear axles are different from each other in a transitionalstatus during turning, the front and rear jack-up forces W_(U) arecompensated effectively, thereby maintaining the vehicle body at apredetermined constant height level over turns.

In the above embodiment, the two lateral acceleration sensors 26a and26b are arranged longitudinally away from each other by a preselecteddistance for projecting lateral accelerations acting on the front andrear axles based on detected acceleration values respectively. However,other suitable measuring means, for example, one lateral accelerationsensor and one yaw rate sensor may be utilized to determine lateralaccelerations acting on the front and rear axles. Additionally, thepressure control valves 20FL to 20RR may be replaced with a flow controlvalve which is operable to effect feedback-control of pressure in thehydraulic cylinder.

Further, the above system of the present invention provides theanti-rolling control gain K_(F) for front suspension units greater thanthe anti-rolling control gain K_(R) for rear suspension units toestablish understeering characteristics, however, the anti-rollingcontrol gains K_(F) and K_(R) may be changed based on desired steeringcharacteristics. For example, a manual gain selector may be arrangedadjacent a driver which serves to vary the anti-rolling control gains tovalues of interest to the driver.

Additionally, the jack-up force W_(U) acting on the outside wheelsduring turning is determined dependent upon an amount of lateral loaddisplacement between the outside and inside wheels and the geometry of asuspension link system, therefore, it is necessary to definecharacteristics, i.e., the magnitude or variation of the neutralpressure command values VN_(F) and VN_(R) provided by the neutralpressure setting circuits 50F and 50R separately for the front and rearsuspension units according to the anti-rolling control gains and/or atype of suspension. For example, if the anti-rolling control gain K_(F)for the front suspension units is set to a greater value, increasingvariation in the neutral pressure command value VN_(F) according tolateral acceleration is advantageously preferable.

What is claimed is:
 1. A method for actively controlling vehicularactive suspension units, said active suspension units being disposedbetween a vehicle body and respective front left, front right, rearleft, and rear right tire wheels, comprising steps of:a) monitoring amagnitude and direction of a lateral acceleration acted upon a vehiclebody; b) providing neutral pressure command values VN_(F) and VN_(R)having voltages varied according to the monitored magnitude anddirection of the lateral acceleration, said neutral pressure commandvalues being decreased as the magnitude of the lateral acceleration isincreased; c) providing gain adjusted values VL_(F) and VL_(R) ofanti-rolling command values according to the magnitude and direction ofthe lateral acceleration, said gains K_(F) and a rear tire wheel sideanti-roll control gain K_(R) and said anti-rolling command values beingmultiplication of both the corresponding anti-roll gain and magnitudeand direction of the lateral acceleration; d) adding the neutralpressure command values VN_(F) and VN_(R) to anti-rolling command valuesVL_(F) and VL_(R) to provide the added values to driving circuits forfront left and rear left suspension units, respectively, and adding theneutral pressure command values VN_(F) and VN_(R) to inverted values ofanti-rolling command values VL_(F) and VL_(R) to provide the addedvalues to driving circuits for front right and rear right suspensionunits, respectively; and e) converting the added values into respectiveenergizing currents i_(FL), i_(FR), i_(RL), and i_(RR), respectively, tobe input to respective pressure control valves of the front left, frontright, rear left and rear right suspension units so that the anti-rolland jacking up suppression controls are carried out for the respectivesuspension units.
 2. A method for actively controlling vehicularsuspension units as set forth in claim 1, wherein a total gain of bothanti-roll gains (K=K_(F) +K_(R)) is set such that a rolling motiongenerated when the lateral acceleration acted upon the vehicle bodygives approximately zero.
 3. A method for actively controlling vehicularsuspension units as set forth in claim 2, wherein K_(F) >K_(R).
 4. Amethod for actively controlling vehicular suspension units as set forthin claim 3, wherein each of the neutral command values VN_(F) and VN_(R)are such that a positive predetermined value VN₀ to receive a burden ofa part of a static weight of the vehicle body is given thereto when themonitored lateral acceleration indicates approximately zero and agradually decreased value VN_(F) or VN_(R) is given when the monitoredlateral acceleration G is increased in the direction of either plus orminus.
 5. A method for actively controlling vehicular suspension unitsas set forth in claim 4, wherein a rate of decrease in the one neutralcommand value VN_(F) is smaller than that in the other neutral commandvalue VN_(R).
 6. A method for actively controlling vehicular suspensionunits as set forth in claim 5, wherein each of said pressure controlvalves comprises a proportional solenoid valve.
 7. A method for activelycontrolling vehicular suspension units as set forth in claim 1, whereinsaid step a) monitors the lateral accelerations acted upon vehicularfront and rear axles of the vehicle body.
 8. A method for activelycontrolling vehicular suspension units as set forth in claim 7, whereinsaid step a) further comprises a step f) calculating one lateralacceleration G_(F) acted upon the vehicular front axle and g)calculating another lateral acceleration G_(R) acted upon the vehicularrear axle, the magnitude and direction of the one lateral accelerationG_(F) being provided for the neutral command values VN_(F) for the frontsuspension units and those of the other lateral acceleration G_(R) beingprovided for the neutral command values VN_(R).